Work over the past decade has yielded many insights into conserved mechanisms in eukaryotic cell cycle control. Central to this regulation are cyclin proteins, activating subunits for cyclin-dependent kinases (CDC). Cdk activity is essential for all eukaryotic cell cycle transitions, and cyclin binding is essential to generate Cdk enzymatic activity, clue to a conformational switch in cyclin-bound Cdk. In addition to this biochemical role of cyclins in Cdk activation, cyclins are also involved in targeting the activated enzyme to appropriate subcellular compartments and likely to bind to specific Cdk phosphorylation targets. These mechanisms may contribute to biological specificity of cyclin action: changing the cyclin partner of a Cdk can in principle result in a highly different spectrum of Cdk-dependent phosphorylation. This would explain why some cyclins, for example, are highly active in inducing DNA replication but inactive at regulating mitosis, while other cyclins have the reverse specificity. A major gap in this picture, and indeed in our understanding of cell cycle control by Calksin general, is the paucity of well-established Cdk phosphorylation substrates with defined roles in cell cycle events. The hypothesis that cyclin-specific phosphorylation targets may bind specifically to the appropriately localized cyclins suggests a powerful use of the phenotypes associated with mis-localized cyclins in an effort to identify cellular regulators and substrates of cyclin/CDK complexes. The long-term objective of this proposal is to use the genetics of budding yeast to determine the relationships between cyclin localization and cyclin function, in an effort to elucidate the regulation and targets of the G1-type cyclin/Cdk complex. It is striking that many problems in human cancer are thought to relate to control of expression and function of humanG1 cyclins such as cyclins D and E. Although not directly homologous to the yeastG1 cyclins, cyclins D and E are thought to play similar roles in activation of the GI/S transcriptional program and allowing activation of S and G2/M cyclins. Elucidation of the mechanisms governing cell cycle progression by Gl-cyclins in budding yeast may significantly impact our understanding of cell cycle progression in higher eukaryotic systems.